System and method for synchronizing phases and frequencies of devices in multi-user, wireless communications systems

ABSTRACT

A system and method for synchronizing the phases and frequencies of devices in multi-user, wireless communications systems are provided. A primary beacon signal is transmitted by a destination node in a wireless communications network to a plurality of source nodes. Secondary beacon signals are also exchanged between the source nodes. Using the primary and secondary beacon signals, the nodes generate local phase and frequency estimates which are used to control local phases and frequencies of the source nodes. The source nodes then transmit common information to the destination at carrier frequencies based on the estimated local frequencies and phases, so that the phases and frequencies of the transmitted information are synchronized to facilitate coherent combining of the bandpass signals at the destination. Phase and frequency synchronization can be applied to wireless communications systems having any number of source nodes, and effects of Doppler shifts and moving platforms are accounted for. Acoustic and radio-frequency signaling can be utilized.

RELATED APPLICATIONS

The present application is a continuation of U.S. Nonprovisional Application Ser. No. 12/555,390, now U.S. Pat. No. 8,634,405. The present application claims the benefit of U.S. Provisional Application Ser. No. 61/191,311, filed on Sep. 8, 2008, the entire disclosure of which is expressly incorporated herein by reference.

GOVERNMENT RIGHTS

This invention was made with government support under Grant Nos. CCF-04-47743, ANI-03-38807 and CNS-06-25637 awarded by the National Science Foundation. The government has certain rights in the invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of wireless communications devices and networks. More specifically, the present invention relates to a system and method for synchronizing phases and frequencies of devices in multi-user, wireless communications systems.

2. Related Art

In multiuser wireless communication systems, the term “distributed beamforming” describes the situation in which two or more separate transmitters with common information work together to emulate an antenna array and to focus their bandpass transmissions toward an intended destination. Distributed beamforming has also been referred to as “collaborative beamforming” and “virtual antenna arrays,” and has also been discussed in the context of coherent cooperative transmission and cooperative multiple-input/multiple-output (MIMO) transmission. In all of these systems, the basic principle is the same: individual sources with common information transmit with phase-aligned carriers such that their bandpass transmissions combine constructively after propagation to the intended destination.

There are several advantages to distributed beamforming. By focusing the transmission toward the intended destination, for instance, less transmit power is needed to achieve a desired signal-to-noise ratio (SNR) target. This feature is particularly appealing in wireless communication systems with energy-constrained nodes, such as sensor networks. In these types of systems, nodes are typically too small to allow for the use of conventional antenna arrays. Distributed beamforming is a powerful technique that offers the potential power gains of conventional antenna arrays to wireless communication systems composed of multiple single-antenna users.

Difficulties with distributed beamforming systems relate to the alignment (synchronization) of phases and frequencies of the transmitters. This difficulty is due, in large part, to the fact that each transmitter in a distributed beamformer has an independent local oscillator which can drift in phase and frequency over time. Transmitters in a distributed beamformer therefore require some method to synchronize their carrier signals so that the bandpass transmissions arrive with reasonable phase and frequency alignment at the intended destination. A precise phase alignment is not critical for beamforming. For example, a two-antenna beamformer with a 30 degree phase offset in the received carriers only suffers a loss with respect to ideal beamforming of approximately 7% of the power in the intended direction. The power gain of a beamformer becomes a power penalty, however, when the carriers arrive at the destination with more than 90 degrees of phase offset. Therefore, there is a need for adequate carrier synchronization to ensure energy-efficient transmission to the destination and to ensure that the sources do not cancel each other's transmissions.

SUMMARY OF THE INVENTION

The present invention relates to a system and method for synchronizing phases and frequencies of devices in multi-user, wireless communications systems. A primary beacon signal is generated by a destination node in a wireless communications network, and transmitted to a set of M source nodes (M being any integer greater than or equal to 2) in the wireless communications network. Using the primary beacon signal, the source nodes generate local phase and frequency estimates. Source node 1 then generates a secondary beacon signal, and transmits the secondary beacon signal to source node 2 at the frequency and phase estimated by the source node 1 using the primary beacon signal. Using the received secondary beacon signal, the source node 2 generates additional local phase and frequency estimates. The secondary beacon signal is then retransmitted from source node 2 to source node 3, and so on, and local phase and frequency estimates are generated at each node, until the beacon is received by source node M. Source node M then generates another secondary beacon signal, and transmits it to source node M−1 at the phase and frequency estimated by the source node M using the first beacon signal. This process is repeated from source node M−1 to source node M−2, and so on, until the retransmitted beacon is received by source node 1. All of the source nodes then transmit common information to the destination at carrier frequencies based on the estimated local frequencies and phases, so that the phases and frequencies of the transmitted information arrive in phase and constructively combine at the destination.

The present invention can be applied to wireless radio frequency communications networks having any number of M>1 stationary or mobile source nodes, each of which is capable only of half-duplex transmission and reception, and none of which have knowledge of their local time or their location within the network. The effects of Doppler shifts and moving platforms are also accounted for, and the present invention can also be applied to acoustic communications networks, i.e., networks of nodes which communicate using acoustic waves.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing features of the invention will be apparent from the following Detailed Description of the Invention, taken in connection with the accompanying drawings, in which:

FIG. 1 is a diagram showing a two-source (M=2) wireless communications network in which the present invention can be implemented to synchronize phases and frequencies;

FIG. 2 is a flowchart showing processing steps according to the present invention for synchronizing phases and frequencies of devices in a two-source (M=2) wireless communication system;

FIGS. 3A-3D are diagrams showing signaling steps implemented by the process shown in FIG. 2;

FIGS. 4-6 are graphs showing performance statistics relating to the present invention;

FIG. 7 is a diagram showing timeslot sequences and associated signaling steps implemented by the present invention;

FIG. 8 is a diagram showing a general M>1 source wireless communications system in which the present invention can be implemented for synchronizing phases and frequencies;

FIG. 9 is an initial carrier phase offset histogram showing the actual phase offset obtained by the present invention;

FIG. 10 is a diagram showing synchronization and beamforming steps of the present invention, taken over time;

FIG. 11 is a diagram showing hardware components of a wireless communications device in which the present invention can be implemented;

FIG. 12 is a schematic diagram showing an acoustic implementation of the present invention;

FIG. 13 is a perspective view of the system shown in FIG. 12;

FIG. 14 is a perspective view showing an acoustic beamforming setup according to the present invention;

FIG. 15 is a diagram showing acoustic beacon signaling implemented by the present invention;

FIG. 16 is a diagram showing phase lock loops established by the acoustic implementation of the present invention;

FIGS. 17A-17E are diagrams showing performance tests of the acoustic implementation of the present invention; and

FIGS. 18-19 are histograms showing beamforming magnitude and energy ratio tests of the acoustic implementation of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a system and method for synchronizing phases and frequencies in multi-user, wireless communications devices, as discussed in detail below in connection with FIGS. 1-19.

FIG. 1 is a diagram showing a two-source (M=2), one-destination system model, indicated generally at 10, of a wireless communications network in which the present invention can be implemented. The network 10 includes two or more source nodes 12 a, 12 b and a destination node 16, each of which could be a wireless communications device such as a cellular telephone, wireless network transceiver, base station, transceiver in a sensor network, personal computer, etc. The nodes 12 a, 12 b and 16 can operate using a suitable, multi-user, wireless communications protocol, such as orthogonal frequency-division, multiple-access (OFDMA) protocol.

Both sources 14 a, 14 b are assumed in the model 10 to have respective isotropic antennas 14 a, 14 b, each of which are single (e.g., non-array) antennas. The node 16 also includes an antenna 18. The channel from node i to node j is modeled as a linear, time-invariant (LTI) system with impulse response g_(ij)(t). Each channel in the system is assumed to be finite-impulse-response (FIR) with delay spread v_(ij). The impulse response of each channel in the system is assumed to be reciprocal in the forward and reverse directions, i.e., g_(ij)(t)=g_(ij)(t), and the noise in each channel is assumed to be Gaussian and white with power spectral density N₀/2. It is assumed that both sources have identical information to transmit to the destination.

FIG. 2 is a flowchart showing processing steps according to the present invention, indicated generally at 20, for synchronizing the phases and frequencies of two sources 14 a, 14 b (M=2) for facilitating virtual antenna beamforming from the sources 14 a, 14 b to the destination 16. Reference is also made to FIGS. 3A-3D, which are diagrams showing the signaling steps carried out by the process of FIG. 2 in various timeslots TS0-TS3 (or, TS₀-TS₃). It is noted that the sources 14 a, 14 b are also referred to herein as sources S1 and S2 (or, S₁ and S₂) and the destination 16 is referred to herein as destination D. Beginning in step 22, the destination D transmits a primary sinusoidal beacon signal 40 to sources S1 and S2 (as shown in FIG. 3A). Then, in step 24, the sources S1 and S2 generate local phase and frequency estimates using the received beacon signal 40. Steps 22 and 24 are carried out in a first timeslot TS0.

The beacon signal 40 could, among other possibilities, be a unit-amplitude sinusoidal primary beacon of duration T₀ from the destination to both sources, x ₀(t)=cos(ω(t−t ₀)+φ₀)tε[t ₀ ,t ₀ +T ₀)  (Equation 1) Under the assumption that all of the channels are single-path, i.e. g_(ij)(t)=α_(ij)δ(t−τ_(ij))∀ij, the signal received at S_(j) in TS0 can be written as: y _(0j)(t)=α_(0j) cos(ω(t ₀+τ_(0j)))+φ₀)+η_(0j)(t) for tε[t₀+τ_(0j),t₀+τ_(0j)+T₀) where η_(0j)(t) denotes the additive white Gaussian noise (AWGN) in the 0→j channel and jε{1,2}. Each source uses its noisy observation from the first timeslot to compute estimates of the received frequency and phase; these estimates are denoted as {circumflex over (ω)}_(0j) and {circumflex over (φ)}_(0j), respectively, at S_(j) for jε{1,2}. We use the usual convention that the phase estimate {circumflex over (φ)}_(0j) is an estimate of the phase of the received signal at the start of the observation at S_(j), i.e. {circumflex over ( φ _(0j) is an estimate of the phase of y_(0j)(t) at time t₀+τ_(0j).

In step 26, the source S1 transmits a secondary sinusoidal beacon signal 42 to S2 using the estimated frequency and initial phase extrapolated in timeslot TS0 (as shown in FIG. 3B). Then, in step 28, source S2 receives the beacon signal 42 and generates local phase and frequency estimates using the received beacon signal 42. Steps 26 and 28 are carried out in a second timeslot TS1.

The timeslot TS1 begins immediately upon the conclusion of the primary beacon y₀₁(t) at S₁. At time t₁=t₀+τ₀₁+T₀, S₁ begins transmitting the sinusoidal secondary beacon signal 42 to S₂ that is a periodic extension of y₀₁(t) (possibly with different amplitude) using the phase and frequency estimates {circumflex over (ω)}₀₁ and {circumflex over (φ)}₀₁. To generate the periodic extension, the frequency estimate {circumflex over (ω)}₀₁ is used to extrapolate the estimated phase of y₀₁(t) at time t₀+τ₀₁ to a phase estimate of y₀₁(t) at time t₁. The extrapolated phase estimate at S₁ at time t₁ can be written as: {circumflex over (φ)}₁={circumflex over (φ)}₀₁+{circumflex over (ω)}₀₁(t ₁−(t ₀+τ₁₀))={circumflex over (φ)}₀₁+{circumflex over (ω)}₀₁ T ₀. The secondary beacon transmitted by S₁ in TS1 can be written as: x ₁₂(t)=a ₁₂ cos({circumflex over (ω)}₀₁(t−t ₁)+{circumflex over (φ)}₁)tε[t ₁ ,t ₁ +T ₁). After propagation through the 1→2 channel, this secondary beacon is received by S₂ as: y ₁₂(t)=α₁₂ a ₁₂ cos({circumflex over (ω)}₀₁(t−(t ₁+τ₁₂))+{circumflex over (φ)}₁)+η₁₂(t) for tε[t₁+τ₁₂,t₁+τ₁₂+T₁) where ζ₁₂(t) denotes the AWGN in the 1→2 channel. From this noisy observation, S₂ generates estimates of the received frequency and phase; these estimates are denoted as {circumflex over (ω)}₁₂ and {circumflex over (φ)}₁₂, respectively.

In step 30, the source S2 transmits a secondary sinusoidal beacon signal 44 to S1 using the estimated frequency and initial phase extrapolated in timeslot TS0 (as shown in FIG. 3C). Then, in step 32, source S1 receives the beacon signal 44 and generates local phase and frequency estimates using the received beacon signal 44. Steps 30 and 32 are carried out in a third timeslot TS2.

The timeslot TS2 begins immediately upon the conclusion of y₁₂(t) at S₂. At time t₂=t₁+τ₁₂+T₁, S2, S₂ begins transmitting the sinusoidal secondary beacon signal 44 to S₁ that is a periodic extension of y₀₂(t) using the phase and frequency estimates {circumflex over (ω)}₀₂ and {circumflex over (φ)}₀₂. Note that S₂'s secondary beacon is a periodic extension of the primary beacon it received in TS0 even though its transmission begins at the conclusion of the secondary beacon received in TS1. Here, S₂ extrapolates the phase estimate {circumflex over (φ)}₀₂ obtained at time t₀+τ₀₂ to time t₂ using the frequency estimate {circumflex over (ω)}₀₂ in order to determine the appropriate initial phase of the secondary beacon signal 44. The extrapolated phase estimate at S₂ at time t₂ can be written as:

$\begin{matrix} {{\hat{\phi}}_{2} = {{\hat{\phi}}_{02} + {{\hat{\omega}}_{02}\left( {t_{2} - \left( {t_{0} + \tau_{02}} \right)} \right)}}} \\ {= {{\hat{\phi}}_{02} + {{{\hat{\omega}}_{02}\left( {\tau_{01} + \tau_{12} - \tau_{02} + T_{0} + T_{1}} \right)}.}}} \end{matrix}$ The secondary beacon signal 44 transmitted by S₂ in TS2 can be written as: x ₂₁(t)=a ₂₁ cos({circumflex over (ω)}₀₂(t−t ₂)+{circumflex over (φ)}₂)tε[t ₂ ,t ₂ +T ₂). After propagation through the 2→1 channel, the secondary beacon signal 44 is received by S₁ as: y ₂₁(t)=α₁₂ a ₂₁ cos({circumflex over (ω)}₀₂(t−(t ₂+τ₁₂))+{circumflex over (φ)}₂)+η₂₁(t) for tε[t₂+τ₁₂,t₂+τ₁₂+T₂) where η₂₁(t) denotes the AWGN in the 1→2 channel and where we have applied the assumption that τ₂₁=τ₁₂ and α₂₁=α₁₂. From this noisy observation, S₁ generates estimates of the received frequency and phase; these estimates are denoted as {circumflex over (ω)}₂₁ and {circumflex over (φ)}₂₁, respectively.

In step 34, which occurs in a fourth timeslot TS3, the sources S1 and S2 transmit signals 46 and 48 simultaneously to destination D (as shown in FIG. 3D) using carrier frequencies based on the estimated local frequencies and extrapolated phases. Specifically, in timeslot TS3, both S₁ and S₂ transmit to the destination as a distributed beamformer with carriers generated as periodic extensions of the secondary beacon signals 42 and 44 received at each source. Since our focus is on the performance of the distributed beamformer in terms of the phase difference of the received signals at the destination, we write the transmissions of S₁ and S₂ as unmodulated carriers. The unmodulated carrier transmitted by S_(j) during TS3 can be written as: x ₁₀(t)=a _(j0) cos({circumflex over (ω)}_(j)(t−t _(3j))+{circumflex over (φ)}_(3j))tε[t _(3j) ,t _(3j) +T ₃)  (Equation 2) where {circumflex over (ω)}_(j) is a frequency estimate at S_(j) that is a function of both {circumflex over (ω)}_(0j) and {circumflex over (ω)}_(ij), i≠j. The extrapolated phase estimates at times t₃₁ and t₃₂ are based on the phase and frequency estimates obtained from the secondary beacon observations and can be written as: {circumflex over (φ)}₃₁={circumflex over (φ)}₂₁+{circumflex over (ω)}₂₁(t ₃₁−(t ₂+τ₁₂)) and {circumflex over (φ)}₃₂{circumflex over (φ)}₁₂+{circumflex over (ω)}₁₂(t ₃₂−(t ₁+τ₁₂)),  (Equations 3 and 4) respectively. As for the transmission start times t₃₁ and t₃₂, S₁ begins transmitting its carrier immediately upon the conclusion of the secondary beacon from S₂, hence t ₃₁ =t ₂+τ₁₂ +T ₂ =t ₀+τ₀₁+2τ₁₂ +T ₀ +T ₁ +T ₂. If S₂ begins transmitting immediately upon the conclusion of its secondary beacon transmission, its carrier will arrive at D earlier than S₁'s carrier. To synchronize the arrivals of the carriers, S₂ should wait for τ_(delay)=τ₀₁+τ₁₂−τ₀₂ after the conclusion of the transmission of its secondary beacon before transmitting its carrier in timeslot TS3. This implies that: t ₃₂ =t ₂ +T ₂+τ_(delay) =t ₀+2τ₀₁+2τ₁₂−τ₀₂ +T ₀ +T ₁ +T ₂. By inspection of FIG. 1, it is noted that τ_(delay) must be non-negative. Moreover, S₂ can directly estimate τ_(delay) by observing the amount of time that elapses from the end of its primary beacon observation in TS0 to the start of its secondary beacon observation in TS1, i.e. t_(delay)=(t₁+τ₁₂)−(t₀+τ₀₂+T₀).

The signal received at D in TS3 can be written as the sum of both carrier transmissions after their respective channel delays, i.e., y ₀(t)=α₀₁ a ₁₀ cos({circumflex over (ω)}₁(t−t ₃)+{circumflex over (φ)}₃₁+α₀₂ a ₂₀ cos({circumflex over (ω)}₂(t−t ₃)+{circumflex over (φ)}₃₂)+η₀(t) for tε[t₃,t₃+T₃) where t₃=t₃₁+τ₀₁=t₃₂+τ₀₂. Standard trigonometric identities can be applied to rewrite y₀(t) as:

$\begin{matrix} {\mspace{79mu}{{{y_{0}(t)} = {{{{a_{bf}(t)}\cos\;\left( {\phi_{bf}(t)} \right)} + {{\eta_{0}(t)}\mspace{14mu} t}} \in \left\lbrack {t_{3},{t_{3} + T_{3}}} \right)}}\mspace{20mu}{where}{{a_{bf}(t)}:=\sqrt{\left( {\alpha_{01}a_{10}} \right)^{2} + \left( {\alpha_{02}a_{20}} \right)^{2} + {2\;\alpha_{01}a_{10}\alpha_{02}a_{20}\cos\;\left( {\phi\;{\Delta(t)}} \right)}}}{{\phi_{bf}(t)}:={{{\hat{\omega}}_{1}\left( {t - t_{3}} \right)} + {\hat{\phi}}_{31} + {\tan^{- 1}\left\lbrack \frac{\alpha_{02}a_{20}\sin\;\left( {\phi\;{\Delta(t)}} \right)}{{\alpha_{01}a_{10}} + {\alpha_{02}a_{20}\cos\;\left( {\phi\;{\Delta(t)}} \right)}} \right\rbrack}}}}} & \left( {{Equation}\mspace{14mu} 5} \right) \end{matrix}$ and where we have defined the carrier phase offset φ_(Δ)(t):=({circumflex over (ω)}₂−{circumflex over (ω)}₁)(t−t ₃)+{circumflex over (φ)}₃₂−{circumflex over (φ)}₃₁ tε[t ₃ ,t ₃ +T ₃). In the special case when carriers arrive at the destination with the same amplitude, i.e. α₀₁a₁₀=α₀₂a₂₀=a, the expressions for a_(bf)(t) and φ_(bf)(t) simplify to: a _(bf)(t)=2a cos(φ_(Δ)(t)/2)tε[t ₃ ,t ₃ +T ₃) φ_(bf)(t)[({circumflex over (ω)}₁+{circumflex over (ω)}₂)(t−t ₃)+{circumflex over (φ)}₃₁+{circumflex over (φ)}₃₂]/2

In step 36, a determination is made as to whether unacceptable phase drift has occurred between sources S1 and S2 during beamforming, which could occur during long transmissions. If so, control returns to step 22, so that timeslots TS0-TS3 can be repeated. Otherwise, processing ends.

The present invention can also be effective in communication systems with multipath LTI channels if minor modifications are made to account for the transient effects of the channels. Such modifications are summarized below.

As with single-path channels, the present invention begins with the transmission of a sinusoidal primary beacon of duration T₀ from the destination D. Since the beacon is of finite duration, the signals received by S₁ and S₂ will have an initial transient component, a steady state component, and a final transient component. It can be shown that the duration of the steady state component at S_(j) is equal to T₀−v_(0j), where v_(0j) denotes the delay spread of channel g_(0j)(t). In order to achieve a steady-state response at both S₁ and S₂, we require T₀>max (v₀₁,v₀₂). The steady-state portion of the beacon received at S_(j) can then be written as: y _(0j)(t)=α_(0j) cos(ωt+φ ₀+θ_(0j))+η_(0j)(t) for tε[t₀+τ_(0j)+v_(0j),t₀+τ_(0j)+T₀) and jε{1,2}. Each source uses only the steady-state portion of its noisy observation in the first timeslot to compute local estimates of the received frequency and phase. The transient portions of the observation are ignored.

The second and third timeslots are as described in the single-path case, with each source transmitting secondary beacons to the other source using the frequency and extrapolated phase estimates obtained from the first timeslot. The phase estimates at each source are extrapolated for transmission of the secondary beacons as periodic extensions of the steady state portion of the primary beacon observations. The only differences with respect to the single path case are that (i) the duration of each secondary beacon must exceed v₁₂=v₂₁ in order to ensure a steady-state observation and that (ii) the sources estimate the received frequency and phase of the secondary beacons using only the steady-state portion of the observations.

Other than the foregoing, no other modifications to the present invention are necessary. In the final timeslot, both sources transmit as in Equation 2 above. Assuming unmodulated carriers, the steady-state signal received at the destination during the final timeslot can be written in the same form as Equation 5 above. The net effect of multipath on the synchronization protocol is that the beacons must be transmitted with durations exceeding the delay spread of the appropriate channels and that the duration of the steady-state observations used for phase and frequency estimation are reduced, with respect to single-path channels, by the delay spread of the multipath channels.

Although the events of the present invention are described in terms of some notion of “true time” t, it is worth mentioning that the present invention does not assume that nodes share a common time reference. An essential feature of the present invention is that, in each of the timeslots TS1, TS2, and TS3, each source transmission is simply a periodic extension of a beacon received in a previous timeslot. No absolute notion of “time-zero” is needed since the phase of a source's transmission is extrapolated from the estimated initial phase of the appropriate beacon observation in a previous timeslot. Moreover, each source transmission in timeslots TS1, TS2, and TS3 is triggered by the conclusion of a beacon in a prior timeslot. The sources do not follow any schedule requiring knowledge of “true time.”

The fact that sources have imperfect local oscillators also implies that local frequency estimates at each source are relative to the source's clock. Suppose, for example, a source's clock t′=μt runs at rate γ with respect to true time t and that the channels are noiseless. A beacon received at true frequency ω rad/sec will appear to this source to be received at frequency {circumflex over (ω)}=ω/γ. Nevertheless, when the source generates a periodic extension of this signal in a later timeslot, the frequency of the transmission will be equal to the product of the local estimate and the local relative clock rate, i.e. ω. Hence, the present invention does not require the sources to share a common time reference, either in terms of clock rate or phase.

It is also noted that, as long as the half-duplex constraint is not violated, the absolute starting and ending times of each of the timeslots are not critical to the performance of the protocol. Since each source transmission in timeslots TS1, TS2, and TS3 is a periodic extension of a beacon received in a prior timeslot, gaps of arbitrary duration can be inserted between the timeslots without directly affecting the phase offset at the destination during beamforming. Gaps between the timeslots may be needed in practical systems, for example, to account for processing time at the sources and/or transient components of beacons received in multipath. In any case, these gaps do not directly affect the relative phase of the periodic extensions since they essentially delay the window in which the periodic extension is transmitted but do not change the phase or frequency of the periodic extension. As a consequence of this property, the estimate of τ_(delay) at S₂ in TS3 is not critical if the beamforming timeslot is sufficiently long. An inaccurate estimate of τ_(delay) only causes S₂'s carrier to begin slightly earlier or later than the carrier of S₁ at D; it does not affect the relative phase of the carriers during beamforming.

The performance of the present invention in terms of the carrier phase offset at the receiver during the beamforming timeslot is now described. In an ideal beamformer, the amplitudes of the received signals add constructively at the destination and a_(bf)(t)=α₀₁a₁₀+α₀₂a₂₀. The non-ideal nature of the distributed beamformer is captured in the carrier phase offset φ_(Δ)(t)=ω_(Δ)(t−t ₃)+φ_(Δ) tε[t ₃ ,t ₃ +T ₃)  (Equation 6) where ω_(Δ):={circumflex over (ω)}₂−{circumflex over (ω)}₁ represents the linear phase drift during beamforming and φ_(Δ):={circumflex over (φ)}₃₂−{circumflex over (φ)}₃₁ represents the initial phase offset at the start of beamforming. Phase and frequency estimation errors at each source result in unavoidable initial carrier phase offset at the start of TS3 as well as linear phase drift over the duration of TS3. As set forth below, a vector notation for the eight estimation errors of the present invention in the illustrative case (M=2) is established. The joint statistics of these errors are analyzed to facilitate statistical description of the carrier phase offset during beamforming.

In the time-slotted round-trip carrier synchronization protocol, each source generates a pair of frequency estimates and a pair of phase estimates from the primary and secondary beacon observations. The estimation error vector is defined as {tilde over (θ)}:=[{tilde over (ω)}₀₁,{tilde over (ω)}₀₂,{tilde over (ω)}₁₂,{tilde over (ω)}₂₁,{tilde over (φ)}₀₁,{tilde over (φ)}₀₂{tilde over (φ)}₁₂,{tilde over (φ)}₂₁]^(T) where {tilde over (ω)}_(0j):={circumflex over (ω)}_(0j)−ω, {tilde over (ω)}_(ij):={circumflex over (ω)}_(ij)−{circumflex over (ω)}_(0i), {tilde over (φ)}_(0j):={circumflex over (φ)}_(0j)−φ₀, and {tilde over (φ)}_(ij):={circumflex over (φ)}_(ij)−{circumflex over (φ)}_(i) for jε{1,2}, iε{1,2}, and i≠j. Note that the frequency and phase estimation errors {tilde over (ω)}_(0j) and {tilde over (φ)}_(0j) are defined with respect to the primary beacon frequency and phase transmitted by D→S_(j). The frequency and phase estimation errors {tilde over (ω)}_(ij) and {tilde over (φ)}_(ij) are defined with respect to the secondary beacon frequency and phase transmitted by S_(i)→S_(j).

To facilitate analysis, it is assumed that the estimation error vector is Gaussian distributed with zero mean and covariance matrix Θ:=E[{tilde over (θ)}{tilde over (θ)}^(T)]. Note that the frequency estimation errors are all independent since (i) observations in different timeslots are affected by independent noise realizations and (ii) observations at S₁ and S₂ are affected by independent noise realizations. This is also true of the phase estimates. The frequency and phase estimates obtained from the same observation, however, are not independent. Hence, all of the off-diagonal elements of the covariance matrix are equal to zero except for the terms cov[{tilde over (ω)}_(ij){tilde over (φ)}_(ij)] for i,jε{1,2}.

The variances on the diagonal of Θ and the covariances on the off-diagonals of Θ can be lower bounded by the Cramer-Rao bound (CRB). Given a sinusoid of amplitude a in white noise with

${{PSD}\frac{N_{0}}{2}},$ the variances and covariance of the frequency and phase estimates can be lower bounded by Equation 8 below:

$\begin{matrix} {{\sigma_{\omega}^{2} \geq \frac{12\;\sigma^{2}f_{s}^{2}}{a^{2}{N\left( {N^{2} - 1} \right)}}}{\sigma_{\phi}^{2} \geq \frac{2\;{\sigma^{2}\left( {{2\; N} - 1} \right)}}{a^{2}{N\left( {N + 1} \right)}}}{{{cov}\left\{ {\overset{\sim}{\omega},\overset{\sim}{\phi}} \right\}} \geq \frac{{- 6}\;\sigma^{2}f_{s}}{a^{2}{N\left( {N + 1} \right)}}}} & \left( {{Equations}\mspace{14mu} 7\text{-}9} \right) \end{matrix}$ where σ² is the variance of the uncorrelated real and imaginary components of the independent, identically distributed, zero-mean, complex Gaussian noise samples, T is the duration of the observation, f_(s) is the sampling frequency, and N is the number of samples in the observation.

Since each source has a pair of unbiased frequency estimates prior to the start of beamforming, the phase drift during beamforming can be reduced by generating the carrier at S_(j) from a linear combination of the local estimates, i.e., {circumflex over (ω)}_(j)=μ_(j){circumflex over (ω)}_(0j)+(1−μ_(j)){circumflex over (ω)}_(ij). In this case, the carrier frequency offset during beamforming can be written as ω_(Δ):={circumflex over (ω)}₂−{circumflex over (ω)}₁=Γ₁ ^(T){tilde over (θ)}  (Equation 10) where Γ₁=[1−μ₁−μ₂,−(1−μ₁−μ₂),1−μ₂,−(1−μ₁),0,0,0,0]^(T).

It can be shown that the carrier frequency offset ω_(Δ) is Gaussian distributed with zero mean for any choice of μ₁ and μ₂ when the frequency estimates are unbiased and Gaussian distributed. A good choice then for the linear combination parameters μ₁ and μ₂ is one that minimizes var[ω_(Δ)]. For jε{1,2} and i≠j, the linear combination parameters that minimize the variance can be determined using standard calculus techniques to be:

$\mu_{j}^{*} = \frac{1}{1 + {\frac{\sigma_{\omega_{ji}}^{2}}{\sigma_{\omega_{ij}}^{2}}\left( \frac{\sigma_{\omega_{01}}^{2} + \sigma_{\omega_{02}}^{2}}{\sigma_{\omega_{01}}^{2} + \sigma_{\omega_{02}}^{2} + \sigma_{\omega_{ji}}^{2}} \right)}}$

From Equations 3 and 4 above, the carrier phase offset at the start of beamforming can be written as: φ_(Δ)=[{circumflex over (φ)}₁₂+{circumflex over (ω)}₁₂(t ₃₂−(t ₁+τ₁₂))]−[{circumflex over (φ)}₂₁+{circumflex over (ω)}₂₁(t ₃₁−(t ₂+τ₁₂))]  (Equation 11) The secondary beacon frequency estimates {circumflex over (ω)}₂₁ and {circumflex over (ω)}₁₂ can be written as: {circumflex over (ω)}₂₁={circumflex over (ω)}₀₂+{tilde over (ω)}₂₁=ω+{tilde over (ω)}₀₂+{tilde over (ω)}₂₁ and {circumflex over (ω)}₁₂={circumflex over (ω)}₀₁+{tilde over (ω)}₁₂=ω+{tilde over (ω)}₀₁+{tilde over (ω)}₁₂ and the phase estimate {circumflex over (φ)}₁₂ can be written as: {circumflex over (φ)}₁₂={circumflex over (ω)}₀₁ T ₀+{circumflex over (φ)}₀₁+{tilde over (φ)}₁₂=(ω+{tilde over (ω)}₀₁)T ₀+φ₀+{tilde over (φ)}₀₁+{tilde over (φ)}₁₂ where {tilde over (φ)}₀₁ is the primary beacon phase estimation error at S₁ and {tilde over (φ)}₁₂ is the secondary beacon phase estimation error at S₂. Similarly, the phase estimate {circumflex over (φ)}₂₁ can be written as

$\begin{matrix} {{\hat{\phi}}_{21} = {{{\hat{\omega}}_{02}\left( {\tau_{01} + \tau_{12} - \tau_{02} + T_{0} + T_{1}} \right)} + {\hat{\phi}}_{02} + {\overset{\sim}{\phi}}_{21}}} \\ {= {{\left( {\omega + {\hat{\omega}}_{02}} \right)\left( {\tau_{01} + \tau_{12} - \tau_{02} + T_{0} + T_{1}} \right)} + \phi_{0} + {\overset{\sim}{\phi}}_{02} + {{\overset{\sim}{\phi}}_{21}.}}} \end{matrix}$ Letting Ψ:=t₃₂−(t₁+τ₁₂)=τ₀₁+τ₁₂−τ₀₂+T₁+T₂ and noting that t₃₁−(t₂+τ₁₂)=T₂, we can plug these results into Equation 11 to get: φ_(Δ)==τ₂ ^(T){tilde over (θ)}  (Equation 12) where Γ₂=[Ψ+T₀, −(Ψ+T₀), Ψ, −T₂, 1, −1, 1, −1]^(T).

Plugging Equations 10 and 12 into Equation 6, the phase offset during beamforming can be expressed in terms of the estimation error vector as: φ_(Δ)(t)=[(t−t ₃)Γ₁+Γ₂]^(T) {tilde over (θ)}tε[t ₃ ,t ₃ +T ₃). Since (t−t₃)Γ₁ and Γ₂ are deterministic and the estimation error vector is assumed to be Gaussian distributed with zero mean, then E[φ_(Δ)(t)]˜

(0,σ_(φ) _(Δ) _((t)) ²) at any tε[t₃,t₃+T₃). The variance of the phase offset can be written as: σ_(φ) _(Δ) _((t)) ²=[(t−t ₃)Γ₁+Γ₂]^(T)Θ[(t−t ₃)Γ₁+Γ₂]  (Equation 13) for any tε[t₃,t₃+T₃). This result can be used to quantify the amount of time that the distributed beamformer provides an acceptable level of carrier phase alignment with a certain level of confidence. At any time tε[t₃,t₃+T₃), the probability that the absolute carrier phase offset is less than a given threshold λ can be written as:

$\begin{matrix} {{{Prob}\;\left\lbrack {{{{\phi\Delta}(t)}} < \lambda} \right\rbrack} = {1 - {2\;{Q\begin{pmatrix} \lambda \\ {\sigma_{\phi\Delta}(t)} \end{pmatrix}}}}} & \left( {{Equation}\mspace{14mu} 14} \right) \end{matrix}$ where Q(x)=_(√2π) ¹∫_(x) ^(∞)exp(−t²/2)dt. The CRB results of Equations 7-9 can be used to provide a lower bound on the variance of the phase offset during beamforming and, as such, an upper bound on Prob[|φ_(Δ)(t)|<λ].

In addition to phase and frequency offsets that occur as a consequence of imperfect estimation, practical oscillators also exhibit phase noise. Oscillator phase noise can cause the phase of each carrier to randomly wander from the intended phase during beamforming and can establish a ceiling on the reliable beamforming time, i.e. the time at which the phase offset between the carriers drifts beyond an acceptable threshold, even in the absence of estimation error.

Oscillator phase noise is denoted as χ_(i)(t) at S_(i) and is modeled as a non-stationary Gaussian random process with zero mean and variance increasing linearly with time from the start of beamforming, i.e. σ_(χ) _(i) ²(t)=r(t−t₃) for tε[t₃,t₃+T₃), where the variance parameter r is a function of the physical properties of the oscillator including its natural frequency and physical type. It is assumed that χ₁(t) and χ₂(t) share the same value of r but are independent phase noise processes. Typical values of r for low-cost radio-frequency oscillators range from 1 to 20 rad²·Hz.

The phase offset during beamforming in terms of the estimation error vector and phase noise processes can be expressed as φ_(Δ)(t)=[(t−t ₃)Γ₁+Γ₂]^(T){tilde over (θ)}+χ₁(t)−χ₂(t) for tε[t₃,t₃+T₃). Note that Γ(t−t₃):=(t−t₃)Γ₁+Γ₂ is deterministic. Under the assumption that the estimates are unbiased, Gaussian distributed, and independent of the mutually independent phase noise processes at each source, we can say that φ_(Δ)(t)˜

(0,σ_(φ) _(Δ) _((t)) ²) where σ_(φ) _(Δ) _((t)) ²=Γ^(T)(t−t ₃)ΘΓ(t−t ₃)+2r(t−t ₃)  (Equation 15) at any tε[t₃,t₃+T₃). This result can be used to quantify the amount of time that the distributed beamformer provides an acceptable level of carrier phase alignment with a certain level of confidence. At any time tε[t₃,t₃+T₃), the probability that the absolute carrier phase offset is less than a given threshold λ can be written as

$\begin{matrix} {{{Prob}\;\left\lbrack {{{{\phi\Delta}(t)}} < \lambda} \right\rbrack} = {1 - {2\;{Q\begin{pmatrix} \lambda \\ {\sigma_{\phi\Delta}(t)} \end{pmatrix}}}}} & \left( {{Equation}\mspace{14mu} 16} \right) \end{matrix}$ where

${Q(x)} = {\frac{1}{\sqrt{2\;\pi}}{\int_{x}^{\infty}{{\exp\left( {{- t^{2}}/2} \right)}\ {{\mathbb{d}t}.}}}}$ The CRB can also be used to provide a lower bound on the variance of the phase offset during beamforming and, as such, an upper bound on the probability that the absolute carrier phase offset is less than a given threshold.

Described now are numerical examples of the present invention in single-path time-invariant channels. The examples in this section assume that all beacons are received at an SNR of

${10\mspace{14mu}{\log_{10}\left( \frac{a^{2}}{2\;\sigma^{2}} \right)}} = {20\mspace{14mu}{dB}}$ and that each channel has a random propagation delay. The primary beacon frequency is ω=2π·900·10⁶ rad/second and the oscillator phase noise variance parameter is assumed to be r=20 rad²·Hz.

The first example shows how the beamforming quality degrades as the duration of the beamforming interval TS₃ increases (for fixed beacon durations) and gives a sense of how often the sources will require resynchronization to maintain acceptable beamforming quality. FIG. 4 plots Prob[|φ_(Δ)(t)|<λ] versus elapsed time from the start of beamforming when the primary beacon duration is fixed at T₀=1 μs and the secondary beacon durations are fixed at T₁=T₂=2 μs. Both sources generate carrier frequencies for beamforming with the optimum linear combining factors μ*₁=μ*₂≈0.5152 in this case. A Monte Carlo simulation with 50,000 iterations was performed to obtain estimates of Prob[|φ_(Δ)(t)|<λ] where, for each new realization of the random parameters, both sources generate maximum likelihood phase and frequency estimates of their noisy observations and use these estimates to generate periodic extensions in the appropriate timeslots. The quality thresholds in FIG. 4 represent the ratio of received power of the distributed beamformer to that of an ideal beamformer in the sense that λ=2 cos⁻¹(u) and |φ_(Δ)(t)|<λ implies that the received power of the distributed beamformer at time t is no worse than u² times that of an ideal beamformer, where 0<u<1. The value of

$u = \frac{1}{\sqrt{2}}$ represents the “break-even” case where the distributed beamformer has the same power efficiency as orthogonal transmission, i.e. the carriers are received with 90 degrees of phase offset.

The results in FIG. 4 show that the carrier phases are closely aligned at the destination with high probability up to t−t₃≈50 μs. By t−t₃=3 ms, however, the probability of having carrier phase alignment such that beamforming is more power efficient than orthogonal transmission is less than ½. Hence, depending on the quality threshold and the confidence in which the threshold must be satisfied, these results show that the distributed beamformer must be periodically resynchronized in order to maintain an acceptable level of performance with high confidence.

In addition to the Monte Carlo simulations, FIG. 4 also plots an upper bound for Prob[|φ_(Δ)(t)|<λ] using the CRB results discussed above to the lower bound set forth in Equation 15. The close match of the experimental and analytical results shows that, as might be expected, the CRB can be used to efficiently predict the performance of the two-source time-slotted round-trip carrier synchronization system when the sources use maximum likelihood phase and frequency estimation. The CRB without phase noise, i.e. r=0, is plotted to also show the effect of phase noise on the expected beamforming time. In these results, since the synchronization timeslots are short, the phase drift due to the phase and frequency estimation errors tends to dominate the effect of the phase noise.

The second example considers the effect of the beacon durations on how long acceptable beamforming quality can be maintained at a desired level of confidence. FIG. 5 plots the 95% confidence beamforming time given a 90%-ideal beamforming quality threshold (λ=2 cos⁻¹ (0.9) and Prob[|φ_(Δ)(t)|<λ]=0.95) using the CRB analytical predictions. All other parameters are identical to FIG. 4. The results in FIG. 5 show two regimes of operation. In the short-beacon regime, the phase and frequency estimation errors dominate the phase noise. In this regime, the 95% confidence beamforming times are approximately flat with respect to the secondary beacon duration when the secondary beacon durations are significantly shorter than the primary beacon duration. When the secondary beacon durations begin to exceed the primary beacon duration, the 95% confidence beamforming times increase at a rate proportional to the secondary beacon durations. If the secondary beacon durations become too long, however, the 95% confidence beamforming time quickly drops to zero. This is due to the fact that the extrapolated phase estimates from the primary beacon become increasingly inaccurate for longer secondary beacon durations.

In the long-beacon regime, the oscillator phase noise dominates the phase and frequency estimation errors. When operating in this regime, the 95% confidence beamforming time is flat with respect to the primary and secondary beacon durations. In FIG. 5, the phase noise establishes a 95% confidence beamforming time ceiling at approximately 5 ms. This ceiling is achieved when T₀=10⁻⁴ or when T₀=10⁻⁵ and T₁=T₂>2·10⁻⁵. These results intuitively suggest that beacon durations should be selected to be long enough such that the estimation error does not dominate but also short enough such that phase noise does not dominate.

FIG. 6 plots the synchronization overhead of the two-source round-trip protocol under the same conditions as in FIG. 5. We define synchronization overhead as the fraction of total time spent synchronizing the sources ignoring propagation delays, i.e.

${{synchronization}\mspace{14mu}{overhead}}:={\frac{T_{0} + T_{1} + T_{2}}{T_{0} + T_{1} + T_{2} + T_{3}}.}$ The beacon durations T₀, T₁ and T₂ are design parameters, but the beamforming duration T₃ is computed from the CRB analytical predictions assuming a 90%-ideal beamforming quality threshold (λ=2 cos⁻¹(0.9)) at 95% confidence. The results in FIG. 6 show that a synchronization overhead of less than 1% can be achieved by selecting beacon durations such as T₀=10⁻⁵ and T₁=T₂=10⁻⁷ or T₀=10⁻⁶ and T₁=T₂=8·10⁻⁶. In both cases, the beacon durations are short enough to avoid operation in the phase-noise dominated regime but also long enough to avoid excessive estimation error. An important conclusion from these results is that the synchronization overhead can be small when the beacon durations are selected such that the phase drift due to estimation error is balanced with the phase noise.

FIG. 7 is a diagram showing the sequence of timeslots TS0-TS3 over time, as carried out by the present invention. As can be seen, synchronization of the phases and frequencies of the sources S1 and S2 first occurs in timeslots TS0-TS2 as described above. Then, in timeslot TS3, beamforming between the sources S1 and S2 to destination D occur, so that information is transmitted from the sources S1 and S2 to the destination D with synchronized phases and frequencies. The labels “PB” and “SB” refer, respectively, to primary and secondary beacon signals. The process is then repeated for subsequent transmissions of information from sources S1 and S2. The process can be implemented as timeslots in any conventional, multi-user wireless communication system, such as an OFDMA wireless system.

The present invention can be implemented in wireless communications systems having any number of M>1 source nodes, to allow for synchronization of phase and frequency of such nodes. Such an arrangement is shown in FIG. 8, which is a model of a multi-user wireless communications system 100 having M source nodes 112 a-112M, each with a respective single antenna 114 a-114M, and a destination node 116 having a single antenna 118. As discussed hereinbelow, a pair of source nodes selected from any of the source nodes 112 a-112M is referred to as source nodes S1 and S2 (or, S₁ and S₂).

As shown in FIG. 8, the channel from node i to node j is modeled as a linear time-invariant (LTI) system with impulse response g_(i,j)(t). The noise in each channel is additive, white, and Gaussian and the impulse response of each channel in the system is assumed to be reciprocal in the forward and reverse directions, i.e., g_(i,j)(t)=g_(j,i)(t). It is assumed that all M sources have identical information to transmit to the destination.

In a distributed beamforming system with M>2 sources, the time-slotted round-trip carrier synchronization protocol has a total of 2M timeslots denoted as TS₀, . . . , TS_(2M−1). The first 2M−1 timeslots are used for the transmission of synchronization beacons and the final timeslot is used for beamforming. The basic concepts of two-source synchronization apply here with additional synchronization timeslots and minor modifications in the calculation of the transmission phase for source nodes S₂, . . . , S_(M−1) shown in FIG. 8. The activity in each timeslot is summarized below:

-   -   1. In TS₀ the destination transmits the sinusoidal primary         beacon to all M sources. Each source generates local phase and         frequency estimates from its observation.     -   2. In TS_(i) for i=1, . . . , M−1, S_(i) transmits a sinusoidal         secondary beacon to S_(i+1). The secondary beacon transmitted by         S_(i) in TS_(i) is a periodic extension of the beacon received         in TS_(i−1). S_(i+1) generates local phase and frequency         estimates from this observation.     -   3. In TS_(M), S_(M) transmits a sinusoidal secondary beacon to         S_(M−1). This secondary beacon is transmitted as a periodic         extension of the primary beacon received by S_(M) in TS₀, with         initial phase extrapolated from the phase and frequency         estimates obtained by S_(M) in TS₀. S_(M−1) generates local         phase and frequency estimates from this observation.     -   4. In TS_(i) for i=M+1, . . . , 2M−2, S_(2M−i) transmits a         sinusoidal secondary beacon to S_(2M−i). The secondary beacon         transmitted by in S_(2M−1) in TS_(i) is a periodic extension of         the secondary beacon received in TS_(i−1). S_(2M−1) generates         local phase and frequency estimates from this observation.     -   5. In TS_(2M−1), all M sources transmit simultaneously to the         destination as a distributed beamformer. The frequency and         initial phase of the carrier transmitted by each source is based         only on the local phase and frequency estimates obtained in the         prior timeslots.

Since, like the two-source case, the total phase shift of the D→S₁→S₂→ . . . →S_(M)→D and the D→S_(M)→S_(M−1) . . . →S₁→D circuits are identical, distributed beamforming between source nodes S₁ and S_(M) can be achieved by following the round-trip protocol and transmitting secondary beacons as periodic extensions of previously received beacons in the same manner as described above. The only difference is that the secondary beacons propagate between S₁ and S_(M) with multiple hops when M>2, rather than via direct propagation in the two-source case. When M>2, however, nodes S₂, . . . S_(M−1) must also derive appropriate transmission phases to participate in the distributed beamformer.

Under the assumption that all of the channels in FIG. 8 are time-invariant, and ignoring estimation errors to ease exposition, the round-trip nature of the protocol and the transmission of periodic extensions implies that the destination will receive carriers from S₁ and S at a phase (relative to the phase of the primary beacon) of: θ^(rt)=θ_(0,1)+θ_(1,2)+ . . . +θ_(M−1,M)+θ_(M,0) where θ_(k,i) denotes the phase of the LTI single-path channel g_(k,i)(t). Let S denote the set of source nodes S_(m) for mεE {2, . . . , M−1}. In order for source node S_(m)εS to transmit a carrier that arrives at the destination with the same phase as S₁ and S_(M), S_(m) must transmit its carrier with phase θ^(rt)−θ_(m,0).

Source node S_(m)εS receives three transmissions during the synchronization phase of the protocol: a primary beacon in TS₀ at phase θ_(0,m)=θ_(m,0), a secondary beacon during the counterclockwise (in the context of FIG. 8, S₁→S₂→ →S_(M) is counterclockwise propagation and S_(M)→S_(M−1) . . . →S₁ is clockwise propagation around the circuit including D) propagation of beacons in TS_(m−1) at phase θ_(m) ^(↓)=θ_(0,1)+θ_(1,2)+ . . . +θ_(m−1,m) and another secondary beacon during the clockwise propagation of beacons in TS_(2M−m−1) at phase θ_(m) ^(↑)=θ_(0,M)+θ_(M,M−1)+ . . . +θ_(m+1,m). Since each node in the system estimates the phase of received beacons relative to its own local time reference, absolute estimates of θ_(m) ^(↑) and θ_(m) ^(↓) at S_(m) will both have an unknown phase offset that depends on the phase of the local time reference at S_(m). To avoid the problem of determining this unknown phase offset, S_(m) can calculate the phase difference between any two phases that were measured under the same local time reference and effectively cancel the offsets. Accordingly, S_(m) can calculate the phase difference between each secondary beacon phase estimate and the primary beacon phase estimate as δ_(m) ^(↑)=θ_(m) ^(↑)−θ_(0,m) δ_(m) ^(↓)=θ_(m) ^(↓)−θ_(0,m).

Since the unknown local phase offset has been canceled in the phase differences δ_(m) ^(↑) and δ_(m) ^(↓), the sum of these terms will also not have any unknown phase offset. Hence, if S_(m) transmits its carrier as a periodic extension of the primary beacon received in TS₀ with an additional phase shift of δ_(m) ^(↑)+δ_(m) ^(↓), the carrier phase of S_(m) can be written as: φ_(m)=θ_(0,m)+δ_(m) ^(↑)+δ_(m) ^(↓)=θ^(rt)−θ_(0,m) which is the desired phase for beamforming since θ_(0,m)=θ_(m,0). After propagation through channel g_(m,0)(t), the carrier from S_(m)εS arrives at the destination with phase θ^(rt) and constructively combines with the carriers from S₁ and S_(m).

In the round-trip synchronization protocol for M>2 sources, we note that the secondary beacons transmitted by S_(m) are only used by S_(m+1) or S_(m−1), depending on the timeslot in which the beacon is transmitted. The other sources in the system (and the destination) ignore these transmissions. While more efficient protocols exploiting the broadcast nature of the wireless links between the sources and requiring less than 2M−1 synchronization timeslots may be possible, the round-trip synchronization protocol possesses the property that the number of synchronization timeslots is linear in M.

As discussed above, although the events of the present invention are described in terms of some notion of “true time” t, it is worth reiterating that the protocol of the present invention does not assume that nodes share a common time reference. In each of the timeslots TS₁, . . . , TS_(M−1), each source transmission is simply a periodic extension of a beacon received in a previous timeslot. In the case of M>2 sources, source nodes S2, . . . , S_(M−1) must also compute a phase offset for their carrier to arrive at the correct phase, but the carriers of these source nodes are each transmitted as a periodic extension of the primary beacon received in TS₀ with a phase shift computed from two local phase differences. No absolute notion of “time-zero” is needed since the phase of each source transmission is extrapolated from the estimated initial phase of the appropriate beacon observation in a previous timeslot. Moreover, each source transmission in timeslots TS₁, . . . , TS_(M−1) is triggered by the conclusion of a beacon in a prior timeslot. The sources do not follow any schedule requiring knowledge of “true time.”

Since each source transmission in the present invention is intended to be a periodic extension of a beacon received in a previous timeslot, it is noted that each source could realize its phase and frequency estimation functions during the synchronization timeslots by using phase-locked loops (PLLs) with holdover circuits. Since beacons are transmitted for only finite durations in the time-slotted protocol, holdover circuits are necessary to prevent tracking of incorrect beacons or noise. We note that each source requires as many PLLs as received beacons in order to “store” all of its local phase and frequency estimates. In the time-slotted synchronization protocol with M>2, this implies that a source may require as many as three PLLs, depending on whether or not it is an “end node”, i.e. S₁ or S_(M), or an “interior node”, i.e., S₂, . . . , S_(M−1).

It is also pointed out that, as long as the half-duplex constraint is not violated, the absolute starting and ending times of each of the timeslots are not critical to the performance of the protocol. Since each source transmission in timeslots TS₁, . . . , TS_(M−1) is a periodic extension of a beacon received in a prior timeslot, gaps of arbitrary duration can be inserted between the timeslots without directly affecting the phase offset at the destination during beamforming. Gaps between the timeslots may be needed in practical systems, for example, to account for processing time at the sources and/or transient components of beacons received in multipath. In any case, these gaps do not directly affect the relative phase of the periodic extensions since they essentially delay the window in which the periodic extension is transmitted but do not change the phase or frequency of the periodic extension. As a consequence of this property for the case when M=2 sources, the estimate of τ_(delay) at S₂ in TS₃ is not critical if the beamforming timeslot is sufficiently long. An inaccurate estimate of τ_(delay) only causes S₂'s carrier to arrive slightly earlier or later than S₁'s carrier at D; it does not affect the relative phase of the carriers during beamforming. The same intuition applies to the case of M>2 sources.

The indirect effect of gaps between the timeslots is that the extrapolated phase estimates will tend to become less accurate as the duration of the gaps increases. This is due to the fact that the gaps cause more time to elapse between the observation and the start of the periodic extension transmission, magnifying the effect of frequency estimation error and/or phase noise. Hence, while the phase offset of the carriers at the destination during beamforming is not directly affected by the absolute starting and ending times of each timeslot, better performance can be achieved when any gaps between the timeslots are minimized.

The effects of mobility (e.g., movement of one or more nodes) and Doppler shift on the performance of the present invention were studied, and techniques for compensating for same are now discussed. Since each of the three channels in the system are accessed in both directions at different times, mobility may lead to phase and frequency offset at the destination during beamforming even in the absence of phase and frequency estimation error at the sources. While the focus here is on the scenario with M=2 sources, the ideas developed in this section also apply to the M>2 source scenario.

In order to isolate the effect of mobility on the two-source distributed beamformer, considered was a system with noiseless single-path channels and perfect oscillators, i.e. no phase noise. In this scenario, the sources are able to perfectly estimate the phase and frequency of their primary and secondary beacon observations. We assume a constant velocity mobility model where, in D's reference frame, S₁ and S₂ are moving with constant velocity v_(i)≦v for iε{1,2}, where v is the maximum source velocity and v/c<<1. While this constant bounded velocity assumption may appear to be somewhat restrictive, it is reasonable in the regime where the synchronization timeslots are short. Any acceleration that occurs in a timeslot will result in a relatively small velocity change if the timeslot is short.

S₁ and S₂ begin transmission of their carriers in TS₃ at times t₃₁ and t₃₂, respectively. The resulting arrival time difference at the destination can be written as

$\begin{matrix} {\tau_{\Delta} = {t_{31} + {\tau_{10}\left( t_{31} \right)} - \left( {t_{32} + {\tau_{20}\left( t_{32} \right)}} \right)}} \\ {= {{\tau_{10}\left( t_{31} \right)} - {\tau_{01}\left( {t_{0} + T_{0}} \right)} + {\tau_{21}\left( {t_{2} + T_{2}} \right)} -}} \\ {{\tau_{12}\left( t_{1} \right)} + {\tau_{02}\left( {t_{0} + T_{0}} \right)} - {\tau_{20}\left( t_{32} \right)}} \end{matrix}$ where τ_(ki)(t) denotes the propagation time of an impulse emitted from transmitter k to receiver i at time t. If the carriers from S₁ and S₂ are both transmitted at frequency ω, the arrival time difference results in a carrier phase offset at the start of beamforming of φ_(Δ)=ωτ_(Δ).

Applying the bound for |τ_(Δ)| developed as discussed below, one can bound the initial carrier phase offset as |φ_(Δ)|≦ωβ[9t _(p)+8T ₁]  (Equation 17) where β:=v/c<<1, t_(p) denotes the maximum propagation delay over all links in the system, and where we have assumed that the secondary beacon durations are identical (T₁=T₂). As a numerical example of the bound, consider a system with a 900 MHz carrier frequency and secondary beacon durations T₁=T₂=2 μs. Suppose the maximum velocity of each source is set to v=100 meters/sec and the maximum propagation time is set to t_(p)=1 μs, corresponding to a maximum distance between any two nodes of 300 meters. From Equation 17, the maximum initial carrier phase offset can be computed to be |φ_(Δ)|≦0.0471 radians, or about 2.7 degrees.

FIG. 9 is an initial carrier phase offset histogram showing the actual phase offset obtained from 20,000 random realizations of the present invention. The analytical bound is also shown in dotted lines.

In addition to an initial carrier phase offset φ_(Δ) at the start of beamforming, mobility may also result in frequency offset ω_(Δ) during beamforming due to Doppler shifts in each link.

Before analyzing the behavior of the round-trip system, we first consider an exchange of short sinusoidal beacons between two nodes denoted by A and B. Suppose B transmits the first beacon. In A's reference frame, B is moving at constant velocity v_(B) and is separating from A at the rate v_(B) cos θ_(BA) where θ_(BA) is the angle of B's motion with respect to the line from A to B. When a beacon is transmitted from A to B, in B's reference frame, the geometry is the same as before except that now A is moving at constant velocity v_(A)=v_(B) and is separating from B at the rate v_(A) cos θ_(AB) where θ_(AB) is defined in the same way as θ_(BA). We can write the ratio of the Doppler shifts as

$\rho:={\frac{{Doppler}\;\left( B\rightarrow A \right)}{{Doppler}\;\left( A\rightarrow B \right)} = \frac{1 + {\frac{\upsilon_{B}}{c}\cos\;\theta_{BA}}}{1 + {\frac{\upsilon_{A}}{c}\cos\;\theta_{AB}}}}$ Since the beacons between A and B were not transmitted simultaneously, it cannot be assumed that θ_(AB)=θ_(BA). Nevertheless, when the elapsed time between transmissions is short, we can expect any difference in the angles to be small. Suppose that we can bound the difference of the angles such that |θ_(BA)−θ_(AB)|≦ε. Then we can bound the range of ρ as

$\rho \in \left\lbrack {\frac{1 - {\frac{\upsilon}{c}\sin\;\left( {\varepsilon/2} \right)}}{1 + {\frac{\upsilon}{c}{\sin\left( {\varepsilon/2} \right)}}} \cdot \frac{1 + {\frac{\upsilon}{c}{\sin\left( {\varepsilon/2} \right)}}}{1 - {\frac{\upsilon}{c}{\sin\left( {\varepsilon/2} \right)}}}} \right\rbrack \Subset {\left\lbrack {\frac{1 - {\beta\;{\varepsilon/2}}}{1 + {\beta\;{\varepsilon/2}}} \cdot \frac{1 + {\beta\;{\varepsilon/2}}}{1 - {\beta\;{\varepsilon/2}}}} \right\rbrack \approx \left\lbrack {{1 - {B\;\varepsilon}},{1 + {\beta\;\varepsilon}}} \right\rbrack}$ where v is the maximum source velocity, β:=v/c<<1, and the approximation results from discarding the insignificant higher order terms in the series representation of the ratio.

In the present invention, beacons are transmitted around the D→S₁→S₂→D and D→S₂→S₁→D_(circuits). The Doppler shift, i.e. the ratio of the output to input frequency, in the g_(k,i)(t) channel is denoted as μ_(k,i). Focusing on propagation through the first circuit, the frequency of the primary beacon received by S₁ can be written as ω₀₁=γ_(0,1)ω. Since the secondary beacon transmitted by S₁ is a periodic extension of this primary beacon, the secondary beacon transmitted by S₁ will be received by S₂ at frequency ω₁₂=γ_(1,2)ω₀₁=γ_(1,2)γ_(0,1)ω. And finally, since S₂ will transmit its carrier as a periodic extension of this secondary beacon, the carrier from S₂ will be received by D at frequency ω₂₀=γ_(2.0)γ_(1.2)γ_(0.1)ω. A similar analysis can be applied to the second circuit.

When the sources are moving with constant velocity, each circuit incurs a composite Doppler shift that is the product of the individual Doppler shifts in each link. If we bound the absolute difference of the angles of all of the forward and reverse links in the round-trip synchronization system by ε, the previous result implies that the ratio of the composite Doppler shifts, i.e.

${\rho_{c}:=\frac{{Doppler}\left( D\rightarrow\left. S_{1}\rightarrow\left. S_{2}\rightarrow D \right. \right. \right)}{{Doppler}\left( D\rightarrow\left. S_{2}\rightarrow\left. S_{1}\rightarrow D \right. \right. \right)}},$ is bounded by:

ρ_(c) ∈ [(1 − βε)²(1 − 2βε), (1 + βε)²(1 + 2βε)] ≈ [1 − 4βε, 1 + 4βε] where we have again discarded the higher order terms in the approximation. Note that the (1±2βε) terms correspond to the ratio of the Doppler shifts for the secondary beacons exchanged between the sources since a source's velocity relative to the other source is bounded by 2v. This result implies that the carrier frequency offset during beamforming can be bounded by: |ω_(Δ)|≦4βεω.

As a numerical example of the bound, consider a system with a 900 MHz carrier frequency, maximum velocity v=100 m/s, and an angle bound ε=0.0175 radians, corresponding to 1 degree of maximum angle difference between all forward and reverse transmissions. In this case, the maximum carrier frequency offset at the destination can be computed to be |ω_(Δ)|≦21 Hz. This amount of carrier frequency offset is likely to be insignificant in most cases when compared to the frequency offset caused by estimation error as well as the phase noise of typical low-cost radio frequency oscillators.

The foregoing results demonstrate that the present invention can be designed such that even significant levels of mobility have little effect on the performance of the distributed beamformer in single-path channels. For a given carrier frequency, the initial carrier phase offset due to mobility can be reduced to a desired level by limiting the range of the links (and hence limiting the maximum propagation time t_(p)), limiting the maximum source velocity, and/or by transmitting short secondary beacons. Similarly, carrier frequency offset during beamforming due to mobility can be reduced by limiting the change in the velocity angles between forward and reverse accesses of a channel. This can also be achieved by limiting the maximum velocity and/or by using short beacon durations.

In systems with mobility, the selection of beacon durations involves a tradeoff between the carrier phase and frequency offsets caused by mobility and the carrier phase and frequency offsets caused by estimation error at each source. Selecting very short beacon durations can minimize the effects of mobility but lead to high levels of estimation error and unnecessary synchronization overhead. On the other hand, very long beacon durations may result in precise phase and frequency estimates at each source, but significant carrier phase and frequency offsets due to mobility as well as operation in the phase-noise dominant regime. As was the case without mobility, beacon durations should be selected to balance the effect of these impairments.

It is noted that the optimum linear combining parameters derived above to minimize var{ω_(Δ)} are, in general, not optimum in systems with Doppler shift. The combining factors μ*₁ and μ*₂ were derived for the case when both frequency estimates {circumflex over (ω)}_(0i), and {circumflex over (ω)}_(ki) at S_(i) are unbiased estimates of ω, as is the case for systems with LTI channels. In systems with time-varying channels and Doppler shifts, however, {circumflex over (ω)}_(0i) and {circumflex over (ω)}_(ki) may have different biases with respect to the primary beacon frequency ω. If these biases are small with respect to the standard deviation of the frequency estimation errors, the optimum combining factors derived above may still be used with little loss of optimality. On the other hand, if the biases due to Doppler effects are large with respect to the standard deviation of the frequency estimation errors, use of the combining factors derived above may significantly degrade the performance of the beamformer. In this case, the carriers should be generated as periodic extensions of the secondary beacons in order to ensure that the ratio of the composite Doppler shifts in each circuit is close to one.

For a time-slotted round-trip distributed beamformer with two sources, the arrival time difference of the start of the carriers at the destination can be written as τ_(Δ)=τ_(Δ) ₀₁ +τ_(Δ) ₂₁ +τ_(Δ) ₀₂   (Equation 18) where τ_(Δ) ₀₁ :=τ₁₀(t₃₁)−τ₀₁(t₀+T₀), τ_(Δ) ₂₁ :=τ₂₁(t₂+T₂)−τ₁₂(t₁), τ_(Δ) ₀₂ =τ₀₂(t₀+T₀)−τ₂₀(t₃₂) and where τ_(ki)(t) denotes the propagation time of an impulse emitted from transmitter k to receiver i at time t. An upper bound on the magnitude of τ_(Δ) can be written as |τ_(Δ)|≦|τ_(Δ) ₀₁ |+|τ_(Δ) ₂₁ |+51 τ_(Δ) ₀₂ |. Before making this bound more explicit, we note that the required accuracy of the forward and reverse propagation times requires consideration of relativistic effects. For example, in a system with 900 MHz beacons/carriers, a error of 100 ps in a propagation time calculation corresponds to a phase error of more than 30 degrees. To maintain consistency and avoid unnecessary transformations between reference frames, we will take all lengths and times in our analysis to be in the destination's reference frame.

Since the speed of light is constant in any reference frame, an impulse emitted by a stationary or moving source in the destination's reference frame always propagates at velocity c from the perspective of the destination. To provide an example of the fundamental calculation that we use to derive the bound, suppose a receiver is distance d₀ from an emitter of an impulse at time t₀ and is moving at constant velocity v on a line directly away from the emitter (all lengths and times are in the destination's reference frame). The emitter can be stationary or moving in the destination's reference frame without affecting the analysis. Since the impulse propagates with velocity c and the receiver is moving directly away from the point at which the impulse was emitted at velocity v, the impulse emitted at t₀ arrives at the receiver when c(t−t ₀)=d ₀ +v(t−t ₀) where the lengths and times are all in destination's reference frame. Hence, the propagation time in the destination's reference frame is

$t_{prop} = {{t - t_{0}} = {\frac{d_{0}}{c - v}.}}$ This expression can also be used to calculate the propagation time of an impulse to a stationary receiver by setting v=0, or to a receiver moving directly toward from the point at which the impulse was emitted under the convention that motion toward the emitter corresponds to negative velocity.

With this example in mind, the first term τ_(Δ) ₀₁ in Equation 18 represents the propagation time difference in the reverse and forward links, respectively, between D and S₁. Since t₃₁>t₀+T₀, this term is maximized when S₁ moves away from D at maximum velocity and is minimized when S₁ moves toward D at maximum velocity. In the former case, we can write

$\begin{matrix} {\tau_{\Delta_{01}}^{+} = {\max\left\lbrack {{\tau_{10}\left( t_{31} \right)} - {\tau_{01}\left( {t_{0} + T_{0}} \right)}} \right\rbrack}} \\ {= {\frac{d_{01} + {v\left( {t_{31} - t_{0}} \right)}}{c} - \frac{d_{01} + {vT}_{0}}{c - v}}} \\ {= \frac{{\beta\left( {t_{31} - t_{0} - T_{0} - {d_{01}/c}} \right)} - {\beta^{2}t_{31}}}{1 - \beta}} \end{matrix}$ where β:=v/c, d₀₁ is the distance from S₁ to D at time t=t₀. Similarly, when S₁ moves toward D at maximum velocity, we can write

$\begin{matrix} {\tau_{\Delta_{01}}^{-} = {\min\left\lbrack {{\tau_{10}\left( t_{31} \right)} - {\tau_{01}\left( {t_{0} + T_{0}} \right)}} \right\rbrack}} \\ {= {\frac{d_{01} - {v\left( {t_{31} - t_{0}} \right)}}{c} - \frac{d_{01} - {vT}_{0}}{c + v}}} \\ {= {\frac{{\beta\left( {t_{31} - t_{0} - T_{0} - {d_{01}/c}} \right)} - {\beta^{2}t_{31}}}{1 + \beta}.}} \end{matrix}$ It can be shown that |τ_(Δ) ₀₁ ⁻|≧|τ_(Δ) ₀₁ ⁺|. Hence, it is evident that

$\begin{matrix} {{\tau_{\Delta_{01}}} \leq \frac{{\beta\left( {t_{31} - t_{0} - T_{0} - {d_{01}/c}} \right)} - {\beta^{2}t_{31}}}{1 + \beta} \leq \frac{{\beta\left( {t_{31} - t_{0} - T_{0}} \right)} + {\beta^{2}t_{31}}}{1 - \beta}} & \left( {{Equation}\mspace{14mu} 19} \right) \end{matrix}$ where the second inequality is applied to allow for a more convenient representation of the overall bound on |τ_(Δ)|.

The second term τ_(Δ) ₂₁ in Equation 18 represents the propagation time difference in the links between S₁ and S₂. Since t₂+T₂>t₁, this term is maximized when the sources are separating at maximum velocity and is minimized when the sources are approaching each other at maximum velocity. In the former case,

$\begin{matrix} {\tau_{\Delta_{21}}^{+} = {\max\left\lbrack {{\tau_{21}\left( {t_{2} + T_{2}} \right)} - {\tau_{12}\left( t_{1} \right)}} \right\rbrack}} \\ {= {\frac{d_{12} + {2\;{v\left( {t_{2} + T_{2} - t_{0}} \right)}}}{c - v} - \frac{d_{12} + {2\;{v\left( {t_{1} - t_{0}} \right)}}}{c - v}}} \\ {= \frac{2{\beta\left( {t_{2} + T_{2} - t_{1}} \right)}}{1 - \beta}} \end{matrix}$ where d₁₂ is the distance from S₁ to S₂ at time t=t₀. Similarly, when the sources are approaching each other at maximum velocity,

$\begin{matrix} {\tau_{\Delta_{21}}^{-} = {\min\left\lbrack {{\tau_{21}\left( {t_{2} + T_{2}} \right)} - {\tau_{12}\left( t_{1} \right)}} \right\rbrack}} \\ {= {\frac{d_{12} - {2\;{v\left( {t_{2} + T_{2} - t_{0}} \right)}}}{c + v} - \frac{d_{12} - {2\;{v\left( {t_{1} - t_{0}} \right)}}}{c + v}}} \\ {= {\frac{{- 2}{\beta\left( {t_{2} + T_{2} - t_{1}} \right)}}{1 + \beta}.}} \end{matrix}$ It can be shown that |τ_(Δ) ₂₁ ⁺|≧|τ_(Δ) ₂₁ ⁻|. Hence, it is evident that

$\begin{matrix} {{\tau_{\Delta_{01}}} \leq {\frac{2{\beta\left( {t_{2} + T_{2} - t_{1}} \right)}}{1 - \beta}.}} & \left( {{Equation}\mspace{14mu} 20} \right) \end{matrix}$

The third term τ_(Δ) ₀₂ in Equation 18 represents the propagation time difference in the forward and reverse links, respectively, between D and S₂. Since t₃₂>t₀+T₀, this term is maximized when S₂ moves toward D at maximum velocity and is minimized when S₁ moves away from D at maximum velocity. In the former case,

$\begin{matrix} {\tau_{\Delta_{02}}^{+} = {\max\left\lbrack {{\tau_{02}\left( {t_{0} + T_{0}} \right)} - {\tau_{20}\left( t_{32} \right)}} \right\rbrack}} \\ {= {\frac{d_{02} - {vT}_{0}}{c + v} - \frac{d_{02} - {v\left( {t_{32} - t_{0}} \right)}}{c}}} \\ {= \frac{{\beta\left( {t_{32} - t_{0} - T_{0} - {d_{02}/c}} \right)} + {\beta^{2}t_{32}}}{1 + \beta}} \end{matrix}$ where d₀₂ is the distance from S₂ to D at time t=t₀. Similarly, when S₂ moves away from D at maximum velocity,

$\begin{matrix} {\tau_{\Delta_{02}}^{-} = {\min\left\lbrack {{\tau_{02}\left( {t_{0} + T_{0}} \right)} - {\tau_{20}\left( t_{32} \right)}} \right\rbrack}} \\ {= {\frac{d_{02} + {vT}_{0}}{c - v} - \frac{d_{02} + {v\left( {t_{32} - t_{0}} \right)}}{c}}} \\ {= {\frac{{\beta\left( {t_{32} - t_{0} - T_{0} - {d_{02}/c}} \right)} + {\beta^{2}t_{32}}}{1 - \beta}.}} \end{matrix}$ It can be shown that |τ_(Δ) ₀₂ ⁺|≧|τ_(Δ) ₀₂ ⁻|. Hence, it is evident that

$\begin{matrix} {{\tau_{\Delta_{02}}} \leq \frac{{\beta\left( {t_{32} - t_{0} - T_{0} - {d_{02}/c}} \right)} + {\beta^{2}t_{32}}}{1 + \beta} \leq \frac{{\beta\left( {t_{32} - t_{0} - T_{0}} \right)} + {\beta^{2}t_{32}}}{1 - \beta}} & \left( {{Equation}\mspace{14mu} 21} \right) \end{matrix}$ where the second inequality is applied to allow for a more convenient representation of the overall bound on |τ_(Δ)|.

Letting t_(p) denote an upper bound on the propagation delay of all links in the system (over all timeslots), t ₃₁ ≦t ₀+3t _(p) +T ₀ +T ₁ +T ₂, t ₃₂ ≦t ₀+4t _(p) +T ₀ +T ₁ +T ₂, and t ₂ +T ₂ −t ₁ ≦t _(p) +T ₁ +T ₂.  (Equations 22-24) Substituting Equations 22-24 in Equations 19-21 and summing the results,

${\tau_{\Delta}} \leq {\frac{{\beta\left( {{9\; t_{p}} + {4\left( {T_{1} + T_{2}} \right)}} \right)} + {\beta^{2}\left( {{7\; t_{p}} + {2\left( {T_{0} + T_{1} + T_{2}} \right)}} \right)}}{1 - \beta}.}$ In the typical case when β<<1 and T₁=T₂, the bound can be approximated as |τ_(Δ)|≦β[9t _(p)+8T ₁].  (Equation 25) This is the result used to bound the initial carrier phase offset at the start of beamforming (Equation 17).

FIG. 10 is a diagram showing the time sequence of synchronization and beamforming events versus beamforming quality. As can be seen, following the synchronization period, beamforming starts with a slight level of phase offset, followed by a period of phase drift (represented by the diagonal line in FIG. 10). As discussed above, the present invention corrects for such phase drift.

FIG. 11 is a schematic diagram showing hardware components of a wireless communications device 50 in which the present invention could be implemented. The device 50 could be a cellular telephone, a cellular base station, a personal computer with a wireless local area network adapter, etc., and could include a radio frequency (RF) or acoustic transceiver 52, a single antenna 54 for allowing RF communication between the transceiver 52 and another device, a microprocessor/digital signal processor 56, a digital-to-analog/analog-to-digital converter (DAC/ADC) 58, a speaker 60, a microphone 62, a non-volatile memory 64 (e.g., read-only memory (ROM), electrically-erasable, programmable ROM (EEPROM), flash memory, or any other suitable, non-volatile computer-readable medium), a random-access memory 68, a an input device 70, and a display 72. A bus 74 permits communication between the foregoing subsystems. In the case of a wireless local area network adapter, the DAC/ADC 58, speaker 60, microphone 62, input device 70, and display 72 need not be provided, and the bus 74 could communicate with a suitable network transceiver 76, such as an Ethernet transceiver. It is noted that the communications device 50 could be implemented at each of the nodes 12 a,-12 b, 16, 112 a-112M, and 116 of FIGS. 1 and 8.

The present invention could be embodied as a software/firmware module in the form of a beamforming engine 66 which is stored in the memory 64 and executed by the microprocessor 56. The engine 66 could be coded in any suitable high- or low-level programming language, and performs the functions discussed herein to optimize beamforming by synchronizing phases and frequencies. The engine 66 controls phase and frequency parameters of the RF transceiver 52, as discussed above, so that the phase and frequency of the transceiver 52 is synchronized with one or more other transceivers to achieve a desired beamforming pattern to a destination.

FIG. 12 is a schematic diagram showing an acoustic node according to the present invention, indicated generally at 120. As mentioned above, the present invention can be implemented using non-RF wireless signals, such as acoustic signals (i.e., signals in the audio range, as well as subsonic and supersonic acoustic signals). The acoustic node 120 could be substituted for each of the RF nodes 12 a, 12 b of FIGS. 1 and 112 a-112M of FIG. 8, discussed above. The acoustic node 120 includes a microphone 122, a digital signal processor (DSP) 124 programmed using DSK code composer studio software, an audio amplifier 128, a speaker 130, and a +5 VDC power supply 126. The DSP 124 could include the TMS320C6713DSK chip manufactured by Texas Instruments, Inc., or any other suitable equivalent. The speaker 130 and the microphone 122 transmit and receive acoustic signals, and could both be substituted with a single acoustic transceiver module.

FIG. 13 is a perspective view of the acoustic node 120. The node 120 includes a housing 132, to which the microphone 122 and speaker 130 are mounted. Optionally, diagnostic buttons and/or lights 134 could be provided. An AC power connection 136 could also be provided (with corresponding power supply equipment within the housing 132, as well as connector 138 for connecting the node 120 to an external computer system (e.g., for diagnostic and/or data retrieval purposes), and a switch 140 for turning the node 120 on and off.

FIG. 14 is a diagram showing two of the acoustic nodes 120 in a beamforming arrangement 140 with a destination node 150. The destination node 150 includes a speaker 152, a microphone 154, an audio recorder module 156, and a computer system 158 (e.g., a laptop computer). The nodes 120 are programmed to execute the phase and frequency synchronization processes of the present invention (in the form of beamforming engines), so as to optimize acoustic beamforming with the destination node 150. Of course, more than two nodes 120 could be provided. A primary beacon signal can be generated by the destination node 150 using the commercially-available Goldware computer program, and the primary beacon signal is received by the nodes 120 and processed as discussed hereinabove. The output from the destination node 150 can also be recorded using the audio recorder 156 at a high sampling rate, e.g., 44,100 samples per second, for analysis of the beamforming pattern received from the nodes 120. Each node 120 includes a 6-th order IIR filter to isolate a 907 Hz tone (which is used as the carrier frequency), as well as an independent local oscillator, which can be generated by multiplying the frequency of the crystal oscillator up to a fixed, nominal frequency. Of course, other carrier frequencies are possible. The 907 Hz carrier frequency was calculated using the following equation, which converts cell-phone carrier frequencies (e.g., of 800 MHz) to acoustic frequencies:

$\lambda = {\frac{v}{f_{RF}} = {\left. \frac{v}{f_{acoustic}}\rightarrow\frac{3\; e\; 8}{800\; e\; 6} \right. = \left. \frac{340}{f_{acoustic}}\rightarrow{f_{acoustic} \approx {907\;{Hz}}} \right.}}$

FIG. 15 is a diagram showing operation stages of the acoustic implementation of the present invention discussed above in connection with FIGS. 12-14. As can be seen, the phase and frequency synchronization process occurs in four stages labeled Stages 1-4. In Stage 1, a 907 Hz tone is transmitted from the destination node (corresponding to the destination node 150, discussed above) to the source nodes (N1-N2, corresponding to the acoustic nodes 120, discussed above), and local phase and frequency estimates are made by the nodes N1 and N2. Then, during Stage 2, the first node N1 transmits the 907 Hz tone to the second node N2 using the phase at which the tone was received at node N1, and the node N2 makes additional local phase and frequency estimates. During Stage 3, the 907 Hz tone is transmitted from the second node N2 to the first node N1, and the node N1 makes additional local phase and frequency estimates. Finally, in Stage 4, both nodes N1 and N2 transmit simultaneously to the destination node using the phase and frequency estimates, so that the acoustic signals arrive at the destination in phase and combine constructively to optimize beamforming to the destination.

FIG. 16 is a diagram showing a phase-locked-loop (PLL) arrangement which can be implemented with the source nodes N1 and N2 and the destination node. As shown in FIG. 16, two PLL signal loops PLL 1 and PLL 2 can be established between the nodes to ensure that the original phase transmitted from the destination node is kept accurately at the source nodes, and that the signals transmitted to the destination will have identical phases.

FIGS. 17A-17E are charts showing performance tests of the acoustic implementation of the present invention. FIG. 17A is an oscilloscope trace showing signaling during Stages 1-4. As can be seen, there are four tones which represent the signals of Stages 1-4, discussed above. The first portion of the trace shows the primary beacon signal during Stage 1. The second and third portions of the trace show the secondary beacon signals exchanged during Stages 2-3. The last portion of the trace shows the beamformed signal transmitted to the destination node in Stage 4. As can be seen, the magnitude of the trace in Stage 4 is almost double the magnitude of the trace in Stages 2-3, indicating that the signals arrived at the destination in nearly perfect phase alignment. FIG. 17B shows the signal of FIG. 17A in more detail, and FIG. 17C shows the behavior of the signal when the speaker of the destination node was turned off. A frequency domain graph of tone over the noise floor is shown in FIG. 17D, and FIG. 17E shows a recording of a quiet room, which was obtained for comparison purposes.

FIGS. 18-19 are histograms showing beamforming magnitude ratio and beamforming energy ratio tests of the acoustic implementation of the present invention, taken after 120 consecutive test runs. The results were analyzed using the following MATLAB code, which took the absolute value of the signal, processed it through a low-order Butterworth filter, and processed a mean value throughout the whole signal while omitting a transient of the filter:

TABLE 1  6 clear all;  7 clc;  8 fs=44100;  9 interval_length = 0.3;  10 high_threshold = 0.2;  11 low_threshold = 0.2;  12  13 x = wavread(‘Z:\500_Research\Test on Server\filtered_pingpong2.wav’);  14 x = x(:,1);  15  16 first_begin = 0;  17 next_begin = 0;  18 k=0;  19 j = 1;  20 i = 1;  21  22 while(j < (length(x)−22*fs))  23 %%%%%%%%%%%%%%%% INPUT BEACON DETECTOR %%%%%%%%%%%%%%%%%%%%  24 first_begin = 0;  25 k = 0;  26 while (first_begin==0)  27 if ((x(j) < low_threshold)) % find the end of the previous beacon  28 k = k+1;  29 else  30 k = 0;  31 end  32 if (k>= 1000)  33 while (first_begin == 0)  34 j = j+1;  35 if (x(j) > high_threshold)  36 first_begin = j;  37 end  38 end  39 end  40 j = j+1;  41 end  42 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%  43 %%%%%%%%%%%%%%%% NODE 1 BEACON DETECTOR %%%%%%%%%%%%%%%%%%%%  44 next_begin = 0; % reset the beginning-of-next-beacon flag  45 k = 0; % reset variable that counts the ‘quiet’ % Input beacon detector  46 while(next_begin==0)  47 if ((x(j) < low_threshold)) % find the end of the previous beacon  48 k = k+1;  49 else  50 k = 0;  51 end  52 if (k >= 1000) % beacon ended, now find the beginning of next beacon Node1 beacon  53 while (next_begin==0)  54 j = j+1;  55 if (x(j) > high_threshold)  56 next_begin = j;  57 end  58 end  59 time = next_begin/44100;  60 a = x((j+0.2*fs) : (j+0.2*fs+interval_length*fs)); % NODE 1 BEACON  61 end  62 j = j+1;  63 end  64 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%  98 if (k >= 1000) % beacon ended, now find the beginning of next beacon Node1 beacon  99 while (next_begin==0) 100 j = j+1; 101 if (x(j) > high_threshold) 102 next_begin = j; 103 end 104 end 105 time = next_begin/44100; 106 c = x((j+0.5*fs) : (j+0.5*fs+interval_length*fs)); % NODE 1+2 BEACON 107 end 108 j = j+1; 109 end 110 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 111 112 j = j+0.5*fs+interval_length*fs; 113 114 a_estimate = envelopePLL(a); 115 b_estimate = envelopePLL(b); 116 c_estimate = envelopePLL(c); 117 118 percentage(i) = c_estimate*100/(a_estimate+b_estimate); 119 energy(i) = 100*((percentage(i)/100. 2); 120 energy(i) 121 i=i+1; 122 123 124 end 125 figure(1) 126 hist(percentage,20); 127 figure(2) 128 hist(energy,20);

The following MATLAB code was also used to generate PLL envelopes:

TABLE 2  6 function [estimate] = envelopePLL(signal)  7  8 % design a low-pass filter.  9 [B,A]=butter(2,0.001); 10 % get length of the signal 11 s_length = length(signal) ; 12 % get power of the signal 13 signal_h = signal. 2; 14 % filter twice. multiply times two. take sqrt 15 envelope_est = sqrt(2*filtfilt(B,A,signal_h)); 16 % figure out the mean over the whole length of the signal 17 estimate = mean(envelope_est(4410;s_length)); 18 19 end

The following formulas were used to find the accuracy ratio of the acoustic implementation of the present invention, where the values A, B, and C represent the magnitudes of the beacons:

${Percentage} = {100*\frac{c}{A + B}}$ ${Energy} = {100*\left( \frac{C}{A + B} \right)^{2}}$

Having thus described the invention in detail, it is to be understood that the foregoing description is not intended to limit the spirit or scope thereof. 

What is claimed is:
 1. A first source node in communication with a destination node and a second source node, comprising: a transmitter; a receiver; a controller in communication with the transmitter and the receiver; and a beamforming engine executable by the controller, wherein execution of the beamforming engine causes the first source node to: process a first beacon signal received from the destination node to estimate local phases and frequencies at the first source node; transmit a second beacon signal to the second source node, the second beacon signal usable at the second source node to estimate local phases and frequencies at the second source node; process a third beacon signal received from the second source node to estimate local phases and frequencies at the first source node; and transmit to the destination node a first carrier signal simultaneously with a second carrier signal transmitted to the destination node by the second source node, the first carrier signal and the second carrier signal containing information common to the first source node and the second source node, wherein phases and frequencies of the first carrier signal are synchronized with phases and frequencies of the second carrier signal to facilitate coherent combining of the common information at the destination node.
 2. The first source node as claimed in claim 1, wherein at least one of the first beacon signal, the second beacon signal, or the third beacon signal is a sinusoidal beacon signal.
 3. The first source node as claimed in claim 1, wherein execution of the beamforming engine further causes the first source node to compensate for Doppler effects prior to transmitting the first carrier signal to the destination node.
 4. The first source node as claimed in claim 1, wherein execution of the beamforming engine further causes the first source node to compensate for motion of at least one of the second source node, the destination node, or at least one third source node prior to transmitting the first carrier signal to the destination node.
 5. The first source node as claimed in claim 1, wherein at least one of the first beacon signal, the second beacon signal, or the third beacon signal is a radio-frequency beacon signal.
 6. The first source node as claimed in claim 1, wherein at least one of the first beacon signal, the second beacon signal, or the third beacon signal is an acoustic beacon signal.
 7. The first source node as claimed in claim 1, wherein the first carrier signal is a radio-frequency signal or an acoustic signal.
 8. A method for synchronizing phases and frequencies of a first source node in communication with a destination node and a second source node, comprising the steps of: receiving, at the first source node, a first beacon signal from the destination node; processing, at the first source node, the first beacon signal to estimate local phases and frequencies at the first source node; transmitting, at the first source node, a second beacon signal to the second source node, wherein the second beacon signal is usable by the second source node to estimate local phases and frequencies at the second source node; receiving, at the first source node, a third beacon signal from the second source node; processing, at the first source node, the third beacon signal to estimate local phases and frequencies at the first source node; and transmitting, at the first source node to the destination node, a first carrier signal simultaneously with a second carrier signal transmitted by the second source node to the destination node, the first carrier signal and the second carrier signal containing information common to the first source node and the second source node, wherein phases and frequencies of the first carrier signal are synchronized with phases and frequencies of the second carrier signal to facilitate coherent combining of the common information at the destination node.
 9. The method as claimed in claim 8, wherein at least one of the first beacon signal, the second beacon signal, or the third beacon signal is a sinusoidal beacon signal.
 10. The method as claimed in claim 8, further comprising the step of compensating, at the first source node, for Doppler effects prior to transmitting the first carrier signal to the destination node.
 11. The method as claimed in claim 8, further comprising the step of compensating, at the first source node, for motion of at least one of the second source node, the destination node, or at least one third source node prior to transmitting the first carrier signal to the destination node.
 12. The method as claimed in claim 8, wherein at least one of the first beacon signal, the second beacon signal, or the third beacon signal is a radio-frequency beacon signal.
 13. The method as claimed in claim 8, wherein at least one of the first beacon signal, the second beacon signal, or the third beacon signal is an acoustic beacon signal.
 14. The method as claimed in claim 8, wherein the first carrier signal is a radio-frequency signal or an acoustic signal.
 15. A non-transitory computer readable medium for synchronizing phases and frequencies of a first source node in communication with a destination node and a second source node, comprising: instruction code for processing, at the first source node, a first beacon signal received from the destination node to estimate local phases and frequencies at the first source node; instruction code for transmitting, at the first source node, a second beacon signal to the second source node, wherein the second beacon signal is usable by the second source node to estimate local phases and frequencies at the second source node; instruction code for processing, at the first source node, a third beacon signal received from the second source node to estimate local phases and frequencies at the first source node; and instruction code for transmitting, at the first source node to the destination node, a first carrier signal simultaneously with a second carrier signal transmitted by the second source node to the destination node, the first carrier signal and the second carrier signal containing information common to the first source node and the second source node, wherein phases and frequencies of the first carrier signal are synchronized with phases and frequencies of the second carrier signal to facilitate coherent combining of the common information at the destination node.
 16. A method for synchronizing phases and frequencies of a first source node in communication with a destination node and a second source node, comprising the steps of: receiving, by the first source node, a first beacon signal from the destination node; locally adjusting, at the first source node, phases and frequencies at the first source node using the first beacon signal; transmitting, at the first source node, a second beacon signal to the second source node, wherein the second beacon signal is usable by the second source node to locally adjust phases and frequencies at the second source node; receiving, at the first source node, a third beacon signal from the second source node; locally adjusting, at the first source node, phases and frequencies at the second source node using the third beacon signal; and transmitting, at the first source node to the destination node, a first carrier signal simultaneously with a second carrier signal transmitted by the second source node to the destination node, the first carrier signal and the second carrier signal containing information common to the first source node and the second source node, wherein phases and frequencies of the first carrier signal are synchronized with phases and frequencies of the second carrier signal to facilitate coherent combining of the common information at the destination node. 